(1) Field of the Invention
The present invention relates to an improvement of synthetic resin insulators comprising a fiber-reinforced plastic rod or pipe (hereinafter, referred to as reinforced plastic rod), an overcoat consisting of an elastic insulating material and metal fittings.
(2) Description of the Prior Art
A reinforced plastic rod produced by impregnating bundles of fibers arranged in their longitudinal direction or knitted fiber bundles with a synthetic resin and bonding the impregnated fiber bundles through the resin has a very high resistance against tensile stress and a very high ratio of strength to weight, but has low weather resistance and tracking resistance. However, elastic insulating materials, such as silicone rubber, ethylene propylene rubber and the like, have excellent weather resistance and tracking resistance. Recently, there have been made various investigations for producing synthetic resin insulators by combining these materials. As a typical synthetic resin insulator, there has been known an insulator comprising a reinforced plastic rod, holding metal fittings fixed to both ends of the rod, and a plurality of overcoats superposed one upon another and fitted with each other such that the overcoats cover the total surface of the reinforced plastic rod located between the holding metal fittings and the outer circumferential portion of the holding metal fitting at its end for receiving the reinforced plastic rod, each of the overcoats consisting of an elastic insulating material, such as ethylene propylene rubber or the like, having a given shape and being provided at its outside with one shed. In this insulator, in order to prevent formation of gaps at the interface between the reinforced plastic rod and the overcoats (hereinafter, interface between a reinforced plastic rod and an overcoat may be merely referred to as interface), or in order to seal the contact portion of adjacent overcoats, a pasty dielectric material, such as silicone grease or the like, is filled in the interface, or the reinforced plastic rod is bonded with the overcoats at the interface and adjacent overcoats are bonded with each other at the contact portion through an adhesive or the like. However, in these insulators, the contact portion of adjacent overcoats is eroded, and the pasty dielectric material filled in the interface leaks out through the contact portion, or water or the like in the external atmosphere penetrates into the interface through the contact portion, and insulation breakdown occurs at the interface, resulting in the breakdown of the insulator.